I'll be on an A320 on monday so I'll see if I notice or can find an answer. I seem to recall a noise like you're talking about on HPs A320s, but I don't fly them often enough to really take notice.

Not to steal your thunder, but nothing can match the annyoning screech/hum of the hydraulics on an MD80 (CO, AS). I know nothing about aircraft, but I asked a pilot once and he said it was hydraulics.